The present invention relates to a liquid level control, and more particularly to a control having a float for sensing the level of a cryogenic fluid to control a metering valve of that cryogenic fluid.
Cryogenic fluids have been used to cool semiconductor components for various reasons. One such reason is that the application of liquid nitrogen to a VLSI CMOS integrated circuit increases the switching speed. This is described in U.S. Pat. No. 4,800,422 by J. Sanwo entitled "A FROSTLESS INTERFACE SUPERCOOLED VLSI SYSTEM." Furthermore, a container for mounting such a supercooled VLSI system is described in U.S. Pat. No. 4,805,420 by Porter et al. entitled "CRYOGENIC VESSEL FOR COOLING ELECTRONIC COMPONENTS." Both of these applications are assigned to NCR Corporation, Dayton, Ohio; and are both hereby incorporated by reference. The problem with the invention disclosed in Porter et al. is that the level of the cryogenic fluid within the cryogenic vessel is difficult to maintain at the level necessary to cool the component mounted therein. The level could be maintained by a free-flow, gravity system, but such systems have difficulties during the filling or other maintenance of the cryogenic fluid reservoir.
Commercial cryogenic fluid level sensors and solenoid controlled cryogenic valves were tested. A fluid logic based sensor had to be immersed at least six inches below the surface of the cryogenic fluid in order to operate properly. In addition to the six inch sensor depth required below the liquid level, a gas collection space is required above the liquid level to gather the evaporated cryogenic liquid. Further, for the most advantageous placement of the solenoid control valve, additional space above the gas collection space is required. Thus, the fluid logic level control proved to require too much space in order to cool a relatively small electronic component.
Another approach uses a vertical array of temperature sensors as the level sensor. Those temperature sensors which are immersed in the cryogenic fluid will all have the same output voltage, while those temperature sensors which are above the level of the cryogenic fluid will have a voltage indicative of a warmer temperature. Thus, by comparing the output voltages from each member of the vertical array the level of the fluid may be sensed. A control circuit to operate a solenoid control valve is straight forward after the level sensor apparatus is decided upon. An initial experiment with such an array yielded a level control system having a solenoid control valve which was constantly turning on and off to regulate the level of an evaporating cryogenic fluid. The constant cycling of the control circuit causes an increased amount of electromagnetic interference in an a region where such interference must be carefully controlled, and the constant cycling of the solenoid valve typically leads to a high wear out rate, which is undesirable from a reliability and maintenance standpoint. A liquid level hysteresis reduces the number and frequency of the solenoid valve cycles, but it requires an increased volume within the cooling enclosure to accommodate the excess cryogenic fluid admitted during each `on` stage to be evaporated during the subsequent `off` stage. The extra volume is undesirable because, as stated previously, it requires too much space to simply cool a relatively small electronic component.
A non-hysteresis system using either multiple solenoid valves, or using a solenoid valve with a variable flow rate, was considered undesirable because of the increased complexity and the increased expense.
It is therefore an object of this invention to provide a mechanical liquid level control for a cryogenic fluid.
It is another object of this invention to provide a mechanical liquid level control for a cryogenic fluid that has a variable flow rate dependent upon the level of the cryogenic fluid.
It is another object of this invention to provide a mechanical liquid level control which is reliable when operated in a cryogenic environment.